Aromatic polycarbonates are excellent not only in mechanical characteristics including impact resistance but also in heat resistance, transparency, and other properties, and have been widely used as engineering plastics in a broad range of fields such as, e.g., bottles for carbonated beverages, electronic bases (CD bases), and transfer belts.
Industrially established processes for producing aromatic polycarbonates include the so-called phosgene process, in which an aromatic diol, e.g., a bisphenol, is reacted with phosgene by interfacial polycondensation.
However, the phosgene process has many disadvantages, e.g., the necessity of use of phosgene, which is toxic to the human body, the inclusion of sodium chloride yielded in a large quantity as a by-product into the polymer produced, the necessity of troublesome wastewater treatment, and the fear of health and air-pollution problems caused by the methylene chloride usually used as a reaction solvent. That is, the phosgene process incurs high equipment costs for taking countermeasures against these health and environmental problems.
The so-called melt polycondensation process or non-phosgene process has also been long known, in which an aromatic polycarbonate is obtained by the interesterification of a diaryl carbonate compound with an aromatic diol compound. This non-phosgene process is generally regarded not only as free of the above-mentioned various problems associated with the phosgene process, but also as advantageous in that an aromatic polycarbonate can be produced at a lower cost. However, the aromatic polycarbonate obtained by the non-phosgene process in which bisphenol A is reacted with diphenyl carbonate generally has a higher content of terminal hydroxyl groups than that obtained by the phosgene process using, for example, bisphenol A, phosgene, a terminal blocker, etc. In addition, a residue of the catalyst used in the non-phosgene process adversely influences the polymer. As a result, the aromatic polycarbonate obtained by the non-phosgene process is generally inferior in heat resistance and hue to that obtained by the phosgene process.
For example, aromatic polycarbonates produced from bisphenol A and phosgene by the phosgene process have a heat resistance of about 500.degree. C. in terms of temperature causing a 5% weight loss on heating (Td5%), which will be described later, whereas aromatic polycarbonates produced by the non-phosgene process generally have a lower heat resistance, sometimes by at least several tens of degrees C., although the heat resistance of the latter polycarbonates varies depending on the kind and amount of the interesterification catalyst used and on the content of terminal hydroxyl groups in the aromatic polycarbonates obtained.
Because molding of aromatic polycarbonates should be conducted at a high temperature around 320.degree. C., polycarbonates having insufficient heat resistance give rise to problems such as cleavage of the polymer backbone, coloration, and a decrease in mechanical strength. In particular, an especially high molding temperature is needed for obtaining a reduced melt viscosity in the case of molding for producing thin-walled (0.3-0.6 mm) hollow containers or injection or extrusion molding for producing articles with complicated shapes. Therefore, in order that an aromatic polycarbonate obtained by the non-phosgene process be put to practical use, improvement in heat resistance and prevention of coloration are much desired.
The interesterification process has another drawback that since the interesterification reaction is performed at high temperatures, undesirable side reactions may take place during the progress of the main reaction depending on the kind of the catalyst used. It is known that as a result of such side reactions, different kinds of units, e.g., phenyl salicylate units, are formed in the polymer backbone. Since the phenyl salicylate structure is a precursor for the dihydroxybenzophenone framework regarded as a cause of hue deterioration, the generation of phenol salicylate structures should be minimized in order to improve hue. If reaction proceeds from these different kinds of units, a branched aromatic polycarbonate results, which is inferior, in optical and mechanical characteristics, to linear aromatic polycarbonates.
Known interesterification catalysts for use in producing aromatic polycarbonates having improved hue in the non-phosgene process include quaternary ammonium or phosphonium salts, e.g., tetraphenylphosphonium tetraphenylboranate, triphenylbutylphosphonium tetraphenylboranate, and tetraphenylphosphonium fluoride (see JP-B-47-17978, JP-A-6-200009, Belgian Patent 675,190, and German Patent 431,239), and boron hydride compounds represented by the formula R'.sub.4 P.BH.sub.n R.sub.4-n (R and R' each is a hydrocarbon group and n is 0 or an integer of 1 to 4) (see U.S. Pat. Nos. 4,330,664 and 5,221,761). (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application", respectively.)
However, while the aromatic polycarbonates obtained with these prior art catalysts have improved hue they have insufficient molecular weight and poor heat resistance or they have a heat resistance (Td5%) as low as 475.degree. to 480.degree. C. while having improved hue and a high molecular weight.
JP-A-6-200009 discloses a technique of using a quaternary ammonium compound or a quaternary phosphonium compound as an interesterification catalyst under specific melt polycondensation conditions to produce an aromatic polycarbonate having a reduced content of different kinds of units including branches.
However, there is a description in the above reference to the effect that use of an interesterification catalyst comprising a combination of any of these onium salts with an alkali metal compound disadvantageously results in a significantly increased content of branches, as demonstrated in Reference Examples 6 and 7 given therein.
The combined use of the quaternary phosphonium compound disclosed in JP-A-6-200009 and an alkali metal compound as an interesterification catalyst is evaluated in Comparative Examples described hereinafter, wherein it is demonstrated that aromatic polycarbonates containing different kinds of units in considerably large amounts (see Comparative Examples 5 and 6) are obtained.
In U.S. Pat. No. 4,363,905, there is a description in Column 6, Table III, Run No. IV-II to the effect that an aromatic polycarbonate having a weight-average molecular weight of 400 and a satisfactory hue was produced by melt-polycondensing bisphenol A with diphenyl carbonate using a combination of Bu.sub.4 PBr and sodium phenolate as an interesterification catalyst. However, such a low-molecular aromatic polycarbonate encounters difficulty in injection molding or extrusion molding. There also is a description in Table II-I, Run No. III to the effect that an aromatic polycarbonate having a weight-average molecular weight of 8,400 and an excellent hue was obtained by melt-polycondensing bisphenol A with bis(o-nitrophenyl) carbonate using the same catalyst system. In Run No. III, because of the use of bis(o-nitrophenyl) carbonate as a starting diaryl carbonate, the material cost is high and heat stability during melt polycondensation is poor, resulting in inclusion of decomposition products into the produced aromatic polycarbonate. Thus, the aromatic polycarbonate obtained has an unsatisfactory hue and reduced mechanical strength, e.g., low impact strength.